Engines can use various types of devices to treat emissions in combustion exhaust gasses, one example of which may be referred to as a catalytic converter. However, performance of a catalytic converter may degraded over time for various reasons, including contamination of various materials occupying catalytic sites needed for reducing exhaust emissions.
One approach to remove contamination of an emission control devices includes operating the device at an elevated temperature and controlling an air-fuel ratio of exhaust gasses in the device. In one example, the air-fuel ratio may be rich, and in another example, the air-fuel ratio may be modulated about stoichiometry. Also, one approach to increase the temperature of the device includes operating some cylinders with a lean air-fuel ratio, and some cylinders with a rich air-fuel ratio, where the exhausts are mixed at, in, or upstream of the device to generate exothermic heat.
The inventors herein have recognized a disadvantage with such an approach. For example, the engine cylinders with different air-fuel ratios may produce different levels of combustion torque. While adjustment of spark retard may be used to reduce this effect, such operation can reduce fuel economy. Alternatively, the cylinders can be operated with different amounts of airflow, however this may require special engine hardware, such as differential valve timing or lift control.
One system to address torque imbalances between cylinders (such as from decontamination operation or other differential torque operation) includes a engine with a first cylinder and a second cylinder operating with different engine torques; a motor coupled to said engine capable of absorbing torque and providing torque; and a controller for varying torque of said motor to compensate for said torque difference so that a total engine and motor torque operates with increased balance.
In this way, it is possible to compensate for torque differences between cylinders without requiring spark retard (although spark may be used in some cases, if desired). Further, torque balance may be achieved even when the cylinders having a torque imbalance have a common valve timing or lift.